1. Technical Field
The present invention relates to devices for processing optical data typical of which are optical pick-up devices for reading data from optical disc memories, and devices for writing (recording or erasing) data in photomagnetic disc memories or reading (playing-back) data therefrom. The photomagnetic disc memories are expected to be put to practical use in a near feature, since data can be written therein in a very high density, read out or erased therefrom. The optical pick-up device is adapted to converge laser light from a semiconductor laser or the like, project the converged light to the data record portion of the optical disc and read data according to the variations of intensity of the light reflected from the portion.
2. Prior Art
With the introduction of optical disc memories of high record density into use in recent years, it is expected to develop efficient, compact and lightweight optical pick-up devices.
Conventional optical pick-up devices consist primarily of an optical system and a drive system.
Basically, the optical system has the function of converging laser light on the record portion of an optical disc by a condenser lens and converting the light reflected from the disc into electric signals by a photodiode. Thus, variations in the amount of reflected light due to the data recorded on the disc are taken out in the form of electric signals.
Such optical systems are divided, according to their action, into isolator optical systems for separating the light reflected from the optical disc from the light impinging on the disc, beam converging optical systems for converging the light to be projected onto the optical disc to a spot of about 1 micron in diameter, and error detecting optical systems for detecting focusing errors or tracking errors. These optical systems comprise a suitable combination of elements such as a semiconductor laser serving as a light source, various lenses, prisms, diffraction gratings, mirrors, quarter wave plates and photodiodes.
Useful drive systems include a focusing drive system, tracking drive system and a radial feed drive system.
The focusing drive system is adapted to maintain a suitable distance between the condenser lens and the optical disc so that the light beam formed by the condenser lens forms a proper spot on the disc surface. Most commonly, the condenser lens is moved axially thereof for adjustment.
The tracking drive system is adapted to cause the laser spot to follow the track of the optical disc without displacement. The mechanisms generally used for this purpose include one for moving the condenser lens perpendicular to the optical axis for adjustment, one for moving the entire optical pick-up head radially of the optical disc for adjustment, and one comprising a pivotable mirror for adjusting the angle of incidence of light on the condenser lens.
The radial feed drive system is a mechanism for feeding the optical pick-up head radially of the optical disc usually by a linear motor.
These conventional optical pick-up devices have the following drawbacks.
The optical system, which is complex, is cumbersome to optically align the components, which are liable to become out of alignment due to vibration.
The device comprises a large number of components, takes much time to assemble and is low in productivity.
The device comprises expensive optical components and is expensive in its entirety.
Use of large optical components renders the device large-sized, while the means needed for holding the optical components make the whole device heavy.
On the other hand, since the photomagnetic disc is capable of being written and read out data, many researches or studies therefor are conducted and some types of record/play-back devices are made on an experimental basis.
The principle of recording or writing data on the photomagnetic disc is as follows: By impinging the light on the record medium such as photomagnetic disc to increase the temperature locally and at the same time by applying the magnetic field thereon, the direction of magnetization of the small area is changed. The section of the area where the temperature is to be increased can be very small to the extent of about 1 micron in diameter, so that it is possible to achive a very high record density. Data can be recorded by applying very week magnetic field, since, in general, magnetic field intensity required to magnetically record data decreases with increase of the temperature of the record medium.
Two methods of recording and erasing are proposed at present time. One is a magnetically modulating method. According to this method, the record medium is irradiated with the laser light constantly and the intensity of the magnetic field applied to the record medium is changed according to data to be written. Another method is called an optically modulating method by which a direct magnetic field is applied to the medium constantly and the laser light to be projected thereto is on-off switched according to data.
It is said that there are a direct optical play-back method and indirect optical play-back method. The former method utilizes such phenomenon that upon impinging a linearly polarized light directly on the record location of the medium, the direction of polarization of the light reflected from (or transmitted through) the portion rotates according to the magnetooptic effect. By the latter method the record pattern on the medium is transferred or copied to a magnetic thin film to read the record.
In any way, the devices for photomagnetically recording and playing-back which are made at present time, especially the playing-back components thereof have the same drawbacks as those of above mentioned conventional optical pick-up devices.